The safe disposal of certain toxic waste is difficult to achieve since removal of the toxins from the waste usually involves complex systems. For example, the fly ash from a coal burning furnace includes toxic levels of mercury, which render the ash unsuitable for subsequent use or easy disposal, such as in a land fill. With land fills becoming over filled, pressure from environmental groups mounting, and legislation directed at stopping many kinds of dumping (such as ocean dumping), incineration of the waste is becoming more popular.
A multiple hearth furnace is conventionally used for treating sludge, garbage, etc., such as from a waste water treatment plant. FIG. 1 shows a counter-flow multiple hearth furnace 100 which usually comprises a feed zone 110 (also serving as a drying zone if the waste to be burned is wet), a combustion zone 120 and a cooling zone 130. Each zone may include a plurality of hearths. The waste, which may be wet, is fed from above through the drying or feed zone 110 so as to be dried by the flue gas, and becomes solidified. The solidified material enters the combustion zone 120 to be burned.
A fan 140 is operable to introduce air through the cooling zone 130 and into the combustion zone 120 from below. In this way the incinerated material from the combustion zone 120 is cooled by the air before exiting the furnace through the output port 131. Exhaust gas exits the drying or feed zone 110 through an exhaust gas port 111.
Almost all materials that are incinerated produce a residual ash. The softening and melting points of the residual ash are important parameters to the combustion operation. In particular, if the temperature of the bed of burning solid material reaches the softening point, of the residual ash, the solid material will become sticky, clog the system, and interfere with rapid incineration of the waste. Therefore, the temperature in the combustion zone 120, especially in the bed of solid material, must be controlled such that it is lower than the melting point, and preferably lower than the softening point, of the residual ash.
A solution to the clogging problem is to cool the combustion zone 120 using large volumes of excess air. However, excess air tends to increase the gas phase oxygen concentration, which increases the burning rate of the solid material, and, thereby the temperature of the bed of the burning solid material. The increased air in the system may also result in other problems, such as extinguished combustion and quenching effect. Another problem occurs in some multiple hearth furnaces when more air is introduced into the system, particularly in furnaces that heat the exhaust gas at the top of the furnaces or are forced to afterburn the entire furnace exhaust gas for pollution control. Indeed, some states require that the exhaust gas exit at a specified temperature, such as 1500 degrees Fahrenheit, which usually means that fuel is added to the gas and ignited just prior to exiting the furnace, or in a subsequent external combustion chamber. Thus, with increased air in the system, more fuel is required to heat or burn the exhaust gas so as to raise the exhaust gas temperature, thereby increasing processing costs.
U.S. Pat. No. 5,957,064 discloses a flue gas recirculation (FGR) system to cool the temperature of the combustion zone. In particular, the flue gas from the drying or feed zone is fed back into the combustion zone through the cooling zone. Typically the bed temperature of an FGR system like this is about 300–500 degrees Fahrenheit lower than that of other furnaces because the recirculating flue gas in the combustion zone helps to limit the peak temperature of both the gas and the solid material. Moreover, introducing flue gas into the combustion zone may decrease the oxygen concentration, thereby decreasing the combustion rate and lowering the solids burning temperature. However, this approach lowers the burning rate, as measured by mass burned per hour per square foot of furnace hearth area, thereby increasing the size of the furnace and cost for a given combustion mass rate.
Therefore, there is a need in the art for an improved method of incinerating waste, which limits the temperature of the combustion zone without sacrificing the solid material burning rate of the furnace.